Single negative resistance tristable operation



NOV. 28, 1967 ABRAHAM 3,355,597

SINGLE NEGATIVE RESISTANCE TRISTABLE OPERATION Filed Nov. 19, 1964 2 Sheets-Sheet 1 F l 6. I

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FIG. 2 a B INVENTOR 25 23 GEORGE ABRAHAM ATTORNEY NOV. 28, 1967 R M 3,355,597

SINGLE NEGATIVE RESISTANCE TRISTABLE OPERATION Filed Nov. 19, 1964 2 Sheets$heet 2 FIG. 3

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ATTORNEY United States Patent 3,355,597 SINGLE NEGATIVE RESISTANCE TRISTABLE OPERATION George Abraham, 3107 Westover Drive SE, Washington, DC. 20020 Filed Nov. 19, 1964, Ser. No. 412,578 7 Claims. (Cl. 30788.5)

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to negative resistance devices and more particularly to single negative resistance devices caused to operate with three stable states.

A simple inexpensive ternary memory element makes possible radix three computers with their greater data handling capabilities. Ternary logic computers have not been considered practical due to the unavailability of a device having a three stable state switching capability at a price comparable to the widely used bistable switching devices of the computers of today.

The device of the present invention is a switching element that is both inexpensive and readily available. This device is a standard negative resistance component caused to oscillate ata frequency high compared to the operating or switching frequency and appropriately loaded to realize three stable state operation. The basic ternary memory element, therefore, is the combination of a normally bistable negative resistance device with a pair of resistors for appropriate loading, a bias source and appropriate reactance to cause oscillation. Trist-able computer operation, with its attendant advantage of increased data handling over the present binary systems, is made available at the minimal expense of the reactance and the two resistors for each switching unit.

It is, accordingly, an object of the present invention to provide an inexpensive ternary memory element.

Another object of the present invention is to provide single negative resistance tristable operation.

It is a further object to provide a simple tristable switch ing element by causing oscillation in a negative resistance device.

A further object of the present invention is to provide a method for converting the common negative resistance device for ternary operation.

Other objects and advantages of the invention will become more fully apparent and better understood from the following description of a preferred embodiment of the invention, as illustrated in the accompanying drawings, in which:

FIG. 1 illustratesan embodiment representative of the FIG. 2 illustrates another embodiment of the present invention;

FIG. 2A shows an alternative tuned circuit for the embodiment of FIG. 2;

FIGS. 3 and 4 show curves corresponding to the FIG. 1 embodiment which aid in understanding the operation of the present invention; and

FIG. 5 illustrates a fabricated circuit embodiment of the present invention.

Referring now to FIG. 1, there is shown a tunnel diode 11, exemplifying the S-type or voltage-controlled negative resistance device, having a series resistor 14 and a parallel Patented Nov. 28, 1967 ance portion of its characteristic curve. Also shown is a tuning capacitor 15 connected directly across the tunnel diode forming a parallel tuned circuit with inductor 12. As will be explained in greater detail below, tristable operation is obtained by making resistor 13 small compared to the negative resistance of the active element to provide astable operation, while oscillation is insured by inductor 12. Series resistor 14, being large compared to the negative resistance, provides a bistable viewing port or load for this ocsillating negative resistance device. With parallel resistance 13 adjusted so that the tunnel diode is loaded down sufficiently such that the magnitude of the oscillation is sufficient to produce a stable region of practical size, the negative resistance portion of the characteristic will have been altered to such a degree as to represent a positive resistance in this region.

Operating as a tristable device, negative pulses applied to the cathode of the tunnel diode will cause the device to switch to successive stable regions. By proper selection of the trigger pulse the negative resistance device can be caused to skip stable regions and switch at a greater rate than one state per pulse. The load selected, as determined by resistors 13 and 14, the bias supply and the characteristic of the negative resistance device all are critical in controlling the switching operation of the tristable element of the present invention. By reversing the polarity of the diode and that of the bais supply 16, positive pulses can be applied to terminal 17 to trigger the circuit. Capacitor 18 serves merely to block DC from the signal source. The output, represented by distinct voltage levels, is realized across resistor 14, between terminal 19 and ground. In the center state the readout may be obtained by either amplitude or frequency measurement.

When the negative resistance device has been trig-gere to the third stable region it must be reset. This can be accomplished in a variety of ways known in the art, such as the application of a positive pulse to terminal 17 or by interrupting the bias to the tunnel diode.

FIG. 2 shows the N-type or current-controlled negative resistance counterpart of the embodiment of FIG. 1. Here the tunnel diode is replaced by an avalanche diode 21, and series resistor 24 and parallel resistor 23 are in the same relative positions and serve the same functions as described above. Bias source 26 also performs the function of its FIG. 1 counterpart. The different configuration of this circuit, however, is attributed to inductor 22 and capacitor 25, here forming a series-tuned circuit across 'its end junctions forward biased in normal operation,

which causes minority carrier injection to produce internal device inductive reactance. The diode isvthus caused to oscillate by shunting capacitance 25 across this device, which function is performed by inductor 12 in the FIG. 1 embodiment. The inductor 22 here performs the same function as the capacitor 15 in the FIG. 1 circuit, that of tuning the circuit. The tunnel diode and voltage-controlled negative resistance devices in general have internal capacitive reactance. Those negative resistance devices of the current-controlled variety, as has been explained for the avalanche diode, exhibit inductive reactance. These internal reactances can be tuned by appropriate external circuit reactance to provide the prerequisite for oscillation. Thus, capacitor 15, in the circuit of FIG. 1, and inductor 22, in the circuit of FIG. 2, so far as these elements designate external circuit reactance, in addition to the internal negative resistance device reactance, are not necessary to the basic functioning of the present invention. As a practical matter, however, these elements provide both stability and selectivity in the frequency of the oscillation desired, and because the values of the added reactances are large compared to their internal counterpart, these reactances are shown as separate elements in the two'circuits.

Capacitor and inductor 22 should be chosen so as to tune the frequency of the oscillation high compared to the desired operating or switching frequency of the circuit, determined by the repetition rate of the incoming trigger pulses, yet below the cut-off frequency of the device. In addition, capacitorlS, which together with inductor 12 determines the oscillating frequency of the tunnel diode, must be large enough to provide a stable operating point in the oscillating or middle stable region of the characteristic curve of this device. This region is designated 11-0 in FIG. 4.

It should be readily understood that the parallel-tuned circuit shown in FIG. 1 is but the preferred embodiment of the tunnel diode circuit and tristability could easily be obtained by the use of a series-tuned circuit, as shown in FIG. 1A, where corresponding parts have corresponding reference characters. Also, the series tuned circuit shown in FIG. 2 could just as readily be supplanted by a parallel-tuned circuit to provide the oscillatory function desired, as shown in FIG. 2A, where corresponding parts have corresponding reference characters. In FIGS. 1A and 2A those portions of the circuits in FIGS. 1 and 2 appearing between points A, B, C and D and A, B, C and D, respectively, are replaced by the configurations shown.

Understanding of the operation of the present invention is facilitated by reference to FIGS. 3 and 4. FIG. 3 shows the current-voltage characteristics for a voltage-controlled negative resistance device, such as the tunnel diode of FIG. I, being subjected to increased values of series resistance. Curve 31 shows the normal negative resistance characteristic. Curve 32 represents a series resistance (re- .sistor 14) equal in value to the negative resistance of the negative resistance device, while curve 33 shows the elfect of a series resistance (resistor 14) large compared to the negative resistance. These curves are to be viewed across the circuit comprising the combined negative resistance device and the series resistor. These series of curves show that as a resistor, connected in series with a voltage-controlled negative resistance device, is increased in value, the slope of the negative resistance portion of the characteristic of this device changes until the series resistor is greater in value than the negative resistance and the negative resistance portion of the curve has then become a positive resistance.

While not shown, the effects of changed values of parallel resistance (resistor 13) on the negative resistance characteristic of a voltage-controlled negative resistance device, such as the tunnel diode, is that as the load on the active element is increased by reducing the parallel resistance, the peak and valley of the characteristic curve is reduced While the slope of the negative resistance portion is decreased. The desired effect of such increased load is astable operation.

In the circuit of this invention, both the internal reactance of the negative resistance device and the external reactance of the circuit cause the load line 42 of FIG. 4 to be nonlinear when switching occurs. The reactance is chosen so that when switching occurs the normally bistable characteristic of the device intersects the negative resistance region. In practice, the value of variable resistor 13 is reduced so that the negative resistance device attempts to pass current through resistor 13. Since the negative resistance device is not a source .of' unlimited power it becomes a poor regulator. This alters the effective negative resistance which increases as the peak-to-valley ratio of the characteristic decreases.

-Hence, upon switching, the intersection of the nonlinear load line is at the negative resistance region to produce astable oscillations, thereby causing additional power to be delivered to the load and further altering the negative resistance characteristic so that an additional region is added.

The above described phenomena applies in a dual sense to the current-controlled negative resistance devices,

such as the avalanche diode. Here, the series resistance is decreased to accomplish the results shown in FIG. 3, while the parallel resistance is increased to accomplish the same results described.

FIG. 4 shows the dynamic operating characteristic of the circuit of FIG. 1. Here, curve 41 represents the dynamic characteristic with positive resistance regions O-a, bc and d-e, separated by negative resistance regions a-b and c-d. Load line 42 occasioned by series resistor.

invention shown in FIG. 1 is now explained in greater detail.

Tunnel diode 11 is caused to oscillate due to inductor 12 forming a tuned circuit with-the internal capacitance of the device. The frequency and stability of the oscillation is controlled by capacitor 15. As the load to the tunnel diode is increased by lowering the value of resistor 13, the peak and valley of the D.C. characteristic of the device as well as the slope of the negative resistance portion will be decreased, as described above. This condition arises when the load has increased to the point of exhausting the tunnel diodes ability to supply power. The increased load, however, has another efiect on the tunnel diode. With the demand for more current from this device, the amplitude of the oscillating current in the non-lineal negative resistance portion of the characteristic increases, bound only by the limits imposed by the peak and valley of the characteristic. This increase in the oscillation in turn increases the distortion of the tunnel diode characteristic, due to internal losses, until a positive resistance region forms in'the negative resistance portion of that curve about the operating or quiescent point resulting in a third stable state. This additional state may be read out by measuring the D.C voltage level or by detecting the oscillating frequency superimposed on this D.C. level. The curve of the tunnel diode now has in fact five diflferent regions of resistance due to the presence of the positive resistance region in the negative resistance portion of that curve, as shown in FIG. 4.

The values of the passive components in the circuit of FIG. 1, resistor 13, inductor 12, capacitor 15 and resistor 14 are limited to an extent by the function performed. Resistor13 must have a value smallcompared to the negative resistance of the tunnel diode to ensure astable operation and to present the dominant load on this negative resistance device. Yet, this resistor must not be so small as to exceed that value of load which would cause the device to burn out. Capacitor 1S and inductor 12 should be selected to provide a frequency of oscillation high compared to the switching frequency of the circuit, yet not higher than the cutolf frequency of the device; This is to provide for a frequency of oscillation so high as to be virtually nonexistent so far as seen by incoming trigger pulses to the circuit so that only the dynamic characteristic of three positive states appears at the operating frequency.

Series resistor 14 is selectedto provide a bistable load to the circuit, ie this resistor should be large compared to the negative resistance of the tunnel diode to provide the proper load for the tristable switching mode.

Thus, when the above described conditions for obtaining three positive resistance regions in the characteristic curve of the negative resistance device have been realized, the series resistor 14 presents a load line transecting these positive resistance regions, as shown in FIG. 4, and provides that load necessary for switching operation.

FIG. illustrates a single substrate microelectronic composite of three tristable memory elements or diodes of the present invention. Each diode comprises an anode 51 and a common cathode provided by the single crystal semiconductor body 56. Three negative resistance junctions are thereby formed where the anodes contact the body 56. Associated with each diode is an inductor 52 and a variable resistor 58, coupling the diodes in a parallel circuit configuration. The inductors are tuned by capacitor 55 connected across the composite device. Resistor 54 is connected in series with the device, While variable resistor 53 is connected in parallel with it, and each performs the same function as resistors 14 and 13 of FIG. 1. Variable resistors 58 alter the characteristic of each tunnel diode element so that one single composite characteristic having seven stable states can be obtained.

The prime asset of the present invention is its teaching of a method of using presently available normally bistable negative resistance devices as tristable elements. The tunnel diode and the avalanche or Shockley diode have been used in illustrating this method. Depending upon the particular use desired, however, any negative resistance device can be used to provide the three stable state operation desired. An added advantage of the present invention is that center state, region b-c in FIG. 4, readout may be obtained by either amplitude or frequency measurement.

The inherent high frequency oscillation capabilities of the tunnel diode enables the elimination of some of the basic circuit elements described. This ability for high frequency oscillation eliminates in many respects the necessity for using an inductance in addition to the inductance provided by the leads of the device itself. In addition, the capacitor 15 need not be an externally added device, as has already been indicated, since the internal capacitance of the device is sufficient for some high frequency use where the absence of accurate control can be tolerated. The ability to do without the tuning reactance applies as readily to the current-controlled negative resistance device embodiment of FIG. 2.

Since various changes and modifications may be made in the practice of the invention herein described without departing from the spirit or scope thereof, it is intended that the foregoing description shall be taken primarily by way of illustration and not in limitation except as may be required by the appended claims.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A multistable switching circuit having at least three stable states and receiving trigger pulses for selecting any desired stable state of said circuit, comprising:

a negative resistance device;

an input circuit connected to said negative resistance device for receiving said trigger pulses;

a first resistor shunting said negative resistance device and having a resistance low compared to the negative resistance of said negative resistance device;

a second resistor in series connection with said negative resistance device selected to have a resistance greater than the negative resistance of said negative resistance device;

a load circuit coupled to said negative resistance device causing said negative resistance device to oscillate when in at least one of said stable states;

and bias means coupled to said negative resistance device by said second resistor to bias said negative resistance device in the negative resistance portion of its characteristic curve.

2. A multistable switching circuit as recited in claim 1, wherein said load circuit is a parallel-tuned inductorcapacitor circuit.

3. A multistable switching circuit as recited in claim 2, wherein said negative resistance device has a voltage-controlled characteristic.

4. A multistable switching circuit as recited in claim 2, wherein said negative resistance device has a currentcontrolled characteristic.

5. A multistable switching circuit as recited in claim 1, wherein said load circuit is a series-tuned inductorcapacitor circuit.

6. A multistable switching circuit as recited in claim 5, wherein said negative resistance device has a voltagecontrolled characteristic.

7. A multistable switching circuit as recited in claim 5, wherein said negative resistance device has a currentcontrolled characteristic.

References Cited UNITED STATES PATENTS 3,054,070 9/1962 Rutz 30788.5 3,076,944 2/ 1963 Watters 30788.5 3,108,233 10/1963 Wasson et a1 307--88.5 3,176,154 3/1965 Salter 307-88.5 3,187,193 6/1965 Rappaport et al 307-885 ARTHUR GAUSS, Primary Examiner.

DAVID J. GALVIN, Examiner.

J. ZAZWORSKY, Assistant Examiner. 

1. A MULTISTABLE SWITCHING CIRCUIT HAVING AT LEAST THREE STABLE STATES AND RECEIVING TRIGGER PULSES FOR SELECTING ANY DESIRED STABLE STATE OF SAID CIRCUIT, COMPRISING: A NEGATIVE RESISTANCE DEVICE; AN INPUT CIRCUIT CONNECTED TO SAID NEGATIVE RESISTANCE DEVICE FOR RECEIVING SAID TRIGGER PULSES; A FIRST RESISTOR SHUNTING SAID NEGATIVE RESISTANCE DEVICE AND HAVING A RESISTANCE LOW COMPARED TO THE NAGATIVE RESISTANCE OF SAID NEGATIVE RESISTANCE DEVICE; A SECOND RESISTOR IN SERIES CONNECTION WITH SAID NEGATIVE RESISTANCE DEVICE SELECTED TO HAVE A RESISTANCE GREATER THAN THE NEGATIVE RESISTANCE OF SAID NEGATIVE RESISTANCE DEVICE; A LOAD CIRCUIT COUPLED TO SAID NEGATIVE RESISTANCE DEVICE CAUSING SAID NEGATIVE RESISTANCE DEVICE TO OSCILLATE WHEN IN AT LEAST ONE OF SAID STABLE STATES; AND BIAS MEANS COUPLED TO SAID NEGATIVE RESISTANCE DEVICE BY SAID SECOND RESISTOR TO BIAS SAID NEGATIVE RESISTANCE DEVICE IN THE NEGATIVE RESISTANCE PORTION OF ITS CHARACTERISTIC CURVE. 